1. Field of the Invention
The present invention relates to a method of manufacturing a surface acoustic wave apparatus to be mounted using metal bumps by a flip chip bonding system and, also relates to the surface acoustic wave apparatus produced by such a method. In particular, the present invention relates to a method of manufacturing a surface acoustic wave apparatus in which at least one electrode for a surface acoustic wave element is formed by a lift-off method and, also relates to the surface acoustic wave apparatus.
2. Description of the Related Art
In recent years, in order to miniaturize surface acoustic wave apparatuses, the surface acoustic wave apparatuses assembled by a flip chip bonding system have been used widely. In this system, bumps made of Au or other material, are formed at electrode pads on a piezoelectric substrate constituting the surface acoustic wave apparatus, and the electrode pads and input and output electrode pads provided on the package or ground electrode pads are electrically connected via the bumps, and are mechanically joined at the same time.
When the aforementioned flip chip bonding system is used, the bumps not only electrically connect the surface acoustic wave apparatus and the package, but also mechanically fix the surface acoustic wave apparatus to the package. Therefore, it is required that the bumps have a high strength. In addition, the joining strength between the bumps and the electrode pads on the piezoelectric substrate must be high, and the adhesion between the electrode pads and the piezoelectric substrate must be high.
In order to increase the joining strength between the electrode pad and the bump, in general, a method, in which the thickness of the electrode pad is sufficiently increased, has been used. In order to increase the thickness of the electrode pad, a conventional method, in which a second electrode layer having a large film thickness is formed on a first electrode layer having a small film thickness, is known.
On the other hand, when the surface acoustic wave apparatus is formed, electrodes for the surface acoustic wave element, for example, an interdigital transducer, reflector, and wiring electrodes, and the aforementioned electrode pads are formed on the piezoelectric substrate. When the electrode pad includes the first and second electrode layers, in many cases, the electrodes for the surface acoustic wave element and the first electrode layer of the electrode pad are formed simultaneously. As the method for forming the electrode for surface acoustic wave element, (1) an etching method or (2) a lift-off method, has been used. In (1) the etching method, a conductive film primarily containing Al is formed over the entire surface of a substrate, and a desired resist pattern is formed by photolithography. Thereafter, the resulting metal film is processed by wet etching or dry etching, and then, the resist is removed. In (2) the lift-off method, the metal film portion adhered on the resist is removed together with the resist and, therefore, the electrode is formed from the remaining metal film portion.
In particular, regarding some surface acoustic wave filters for use in a 800 MHz band or in a 1 GHz to 2 GHz band, surface acoustic wave apparatuses are formed by the use of the aforementioned (2) lift-off method. An example of the method for manufacturing the aforementioned surface acoustic wave apparatus will now be described with reference to FIGS. 22 to 24.
As shown in FIG. 23A, a resist pattern 102 is formed on a piezoelectric substrate 101 by photolithography. A metal film 103 primarily containing Al is formed on the piezoelectric substrate 101 as shown in FIG. 23B. Subsequently, the resist pattern 102 is removed together with the metal film portion adhered thereon by a lift-off process. Thus, a first electrode layer 103a for constituting an electrode pad and an electrode for the surface acoustic wave element 103b are simultaneously formed on the piezoelectric substrate 101 as shown in FIG. 23C. Then, a resist pattern 104 is formed (FIG. 23D). A metal film 105 is formed as shown in FIG. 24A, and the resist pattern 104 is removed by performing the lift-off process again. Consequently, as shown in FIG. 24B, a second electrode layer 105a is formed on the first electrode layer 103a and, therefore, electrode pads 106 having a double-layer structure can be produced.
Next, as shown in FIG. 22, bumps 107 are joined onto the electrode pads 106. A surface acoustic wave apparatus 108 is joined with a package by a flip chip bonding system using the bumps 107.
Regarding the above-described prior art shown in FIGS. 22 to 24, in the case where the first electrode layer 103a of the electrode pad 106 was formed by the lift-off method, since the adhesion between the piezoelectric substrate 101 and the first electrode layer 103a was relatively weak due to the effects of the resist used for the lift-off, when the formation was performed using bumps 107 by a wire bump bonding method concurrently using ultrasonic waves and heat, sometimes, peeling occurred between the first electrode layer 103a and the piezoelectric substrate 101.
Furthermore, when the surface acoustic wave apparatus 108 was mounted on the package by the flip chip bonding system, and airtight sealing was performed by a covering member, sometimes cracks occurred in the piezoelectric substrate 101 in areas adjacent or near the electrode pads 106 due to the mechanical stress brought about by the residual stress. Therefore, the reliability, especially the reliability of the mechanical strength, of the surface acoustic wave apparatus was significantly degraded.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method of manufacturing a surface acoustic wave apparatus and a surface acoustic wave apparatus produced by such a method, the apparatus having superior reliability, and in which the adhesion between the electrode pad and the piezoelectric substrate is very high, peeling of the electrode pad from the piezoelectric substrate is prevented from occurring, and cracks are prevented from being generated during the mounting on the package by the flip chip bonding system.
In addition, preferred embodiments of the present invention provide a method of manufacturing a surface acoustic wave apparatus and a surface acoustic wave apparatus, the apparatus having superior reliability in the electrical connection of electrode pads and electrodes for surface acoustic wave element with the wiring electrodes.
In accordance with preferred embodiments of the present invention, a surface acoustic wave apparatus is preferably a type that is mounted via bumps by a flip chip bonding system.
According to a first preferred embodiment of the present invention, a method of manufacturing a surface acoustic wave apparatus includes the steps of preparing a piezoelectric substrate, forming a first electrode layer of an electrode pad on the piezoelectric substrate, forming at least one electrode for a surface acoustic wave element after the step of forming the first electrode layer, forming a second electrode layer of the electrode pad after the step of forming the electrode for the surface acoustic wave element, and forming a wiring electrode for electrically connecting the electrode pad and the electrode for surface acoustic wave element. Therefore, an electrode forming process that is ideal for forming each of the electrode pad and the surface acoustic wave element is provided.
Regarding the method of manufacturing according to the first preferred embodiment of the present invention, the wiring electrode for electrically connecting the second electrode layer and the electrode for surface acoustic wave element may be simultaneously formed with the second electrode layer. In this case, since the wiring electrode can be simultaneously formed with the second electrode layer, the reliability in the electrical connection between the electrode pad and the wiring electrode can be increased, and the simplification of the manufacturing steps can be achieved.
The method of manufacturing according to the first preferred embodiment of the present invention preferably includes the step of forming an adhesive layer as a substrate prior to the formation of the wiring electrode and the second electrode layer, wherein the wiring electrode and the second electrode layer are preferably made of Al or Al alloy, and the adhesive layer is preferably made of metal or an alloy having adhesion to the first electrode layer higher than that of Al or Al alloy.
By forming the aforementioned adhesive layer as the substrate layer, the adhesion of the second electrode layer to the first electrode layer can be further increased and, therefore, peeling of the electrode pad from the piezoelectric substrate and occurrence of cracks in the piezoelectric substrate during formation of the bumps and during mounting by the flip chip bonding system are reliably prevented.
The method of manufacturing according to the first preferred embodiment of the present invention may further include the step of performing etching in order to make stepwise end surfaces at joint portions, to be connected with the wiring electrodes, of the electrode for the surface acoustic wave element and the electrode pad, after the step of forming the electrode for surface acoustic wave element, wherein the wiring electrode, for electrically connecting the electrode for surface acoustic wave element and the first electrode layer of the electrode pad, and the second electrode layer of the electrode pad may be simultaneously formed from the same conductive film. Consequently, the reliability of the electrical connection between the wiring electrodes and the electrode for the surface acoustic wave element and the electrode pad is greatly increased.
Regarding the method of manufacturing according to the first preferred embodiment of the present invention, each of the electrode for the surface acoustic wave element and the first electrode layer of the electrode pad may have at least two end surfaces of the joint portion and, therefore, the reliability in the electrical connection is even more increased.
Regarding the method of manufacturing according to the first preferred embodiment of the present invention, the electrode for the surface acoustic wave element to be connected with the electrode pad may be formed such that the end surface of the electrode for surface acoustic wave element is in contact with the first electrode layer of the electrode pad in the step of forming the electrode for the surface acoustic wave element. In this case, since the end surface of the electrode for the surface acoustic wave element to be connected with the electrode pad is formed to contact with the first electrode layer of the electrode pad, the reliability of the electrical connection between the wiring electrodes and the electrode for the surface acoustic wave element and the electrode pad is greatly increased.
Regarding the method of manufacturing according to the first preferred embodiment of the present invention, a particle diameter of a conductive particle in the conductive film constituting the wiring electrode and the second electrode layer is preferably smaller than a particle diameter of a conductive particle in one of the electrode for the surface acoustic wave element and the first electrode layer, which has a smaller film thickness. Consequently, the reliability of the electrical connection is even more increased.
Regarding the method of manufacturing according to the first preferred embodiment of the present invention, the electrode for the surface acoustic wave element may be preferably formed by a lift-off method, and the first electrode layer of the electrode pad may be preferably formed by etching. In this case, the electrode for the surface acoustic wave element and the first electrode layer of the electrode pad can be formed with high precision, and furthermore, occurrence of cracks in the piezoelectric substrate at areas near the electrode pad can be prevented even more reliably.
Regarding the method of manufacturing according to the first preferred embodiment of the present invention, at least one electrode for a second surface acoustic wave element that is different from the aforementioned electrode for the surface acoustic wave element may be simultaneously formed with the first electrode layer during the step of forming the first electrode layer. In this case, since the electrode for the second surface acoustic wave element having a thickness different from that of the aforementioned electrode for surface acoustic wave element can be formed very easily, surface acoustic wave elements having different characteristics can be formed with ease on the piezoelectric substrate.
According to a second preferred embodiment of the present invention, a surface acoustic wave apparatus is provided. The surface acoustic wave apparatus includes a piezoelectric substrate, at least one electrode for a surface acoustic wave element disposed on the piezoelectric substrate, an electrode pad disposed on the piezoelectric substrate and arranged to be joined with a bump, and a wiring electrode for electrically connecting the electrode pad and the electrode for the surface acoustic wave element, wherein the electrode pad includes a first electrode layer disposed on the piezoelectric substrate and a second electrode layer laminated on the first electrode layer, the first electrode layer is formed by etching of a metal film, and at least one electrode for the surface acoustic wave element is formed by a lift-off method. Therefore, peeling of the electrode pad from the piezoelectric substrate and occurrence of cracks in the piezoelectric substrate due to the external force applied during the formation of the bumps and during the mounting on the package by the flip chip bonding system or other suitable system, is reliably prevented.
Regarding the surface acoustic wave apparatus according to the second preferred embodiment of the present invention, the wiring electrode and the second electrode layer may be integrally formed from the same metal film. In this case, the reliability of the electrical connection between the electrode pad and the wiring electrode can be increased, and the manufacturing steps are greatly simplified.
The surface acoustic wave apparatus according to the second preferred embodiment of the present invention may further include an adhesive layer as a substrate for the wiring electrode and the second electrode layer, wherein the wiring electrode and the second electrode layer are preferably made of Al or Al alloy or other suitable material, and the adhesive layer is preferably made of metal or alloy or other suitable material having adhesion to the first electrode layer higher than that of Al or Al alloy. Since the aforementioned adhesive layer is formed as the substrate layer, the adhesion of the second electrode layer to the first electrode layer can be further increased and, therefore, peeling of the electrode pad from the piezoelectric substrate and occurrence of cracks in the piezoelectric substrate during formation of the bumps and during mounting by the flip chip bonding system are prevented even more reliably.
According to a third preferred embodiment of the present invention, another surface acoustic wave apparatus to be mounted via bumps by a flip chip bonding system is provided. The surface acoustic wave apparatus according to this preferred embodiment preferably includes a piezoelectric substrate, at least one electrode for a surface acoustic wave element disposed on the piezoelectric substrate, an electrode pad disposed on the piezoelectric substrate and arranged to be joined with the bump, and a wiring electrode for electrically connecting the electrode pad and the electrode for surface acoustic wave element, wherein the electrode pad includes a first electrode layer disposed on the piezoelectric substrate and a second electrode layer laminated on the first electrode layer, the second electrode layer and the wiring electrode are integrally formed from the same conductive film, and end surfaces of joint portions, to be electrically connected with the wiring electrodes, of the first electrode layer and the electrode for surface acoustic wave element are arranged in a stepwise configuration. Therefore, the reliability of the electrical connection of the wiring electrodes between the electrode pad including the first and second electrode layers and the electrode for the surface acoustic wave element is effectively increased.
Regarding the surface acoustic wave apparatus according to the third preferred embodiment of the present invention, each of the electrode for the surface acoustic wave element and the first electrode layer of the electrode pad may have at least two end surfaces of the joint portion. In this case, the reliability of the electrical connection between the electrode for the surface acoustic wave element and the first electrode layer of the electrode pad is further increased.
According to a fourth preferred embodiment of the present invention, another surface acoustic wave apparatus to be mounted via bumps by a flip chip bonding system is provided. The surface acoustic wave apparatus according to this preferred embodiment preferably includes a piezoelectric substrate, at least one electrode for a surface acoustic wave element disposed on the piezoelectric substrate, an electrode pad which is disposed on the piezoelectric substrate and is arranged to be joined with the bump, and a wiring electrode for electrically connecting the electrode pad and the electrode for surface acoustic wave element, wherein the electrode pad includes a first electrode layer disposed on the piezoelectric substrate and a second electrode layer laminated on the first electrode layer, the second electrode layer and the electrode for the surface acoustic wave element are integrally formed from the same conductive film, and the electrode for the surface acoustic wave element and the first electrode layer of the electrode pad, to be connected with the electrode for the surface acoustic wave element, are arranged in contact with each other. Therefore, the reliability of the electrical connection between the electrode for the surface acoustic wave element and the electrode pad is greatly increased.
Regarding the surface acoustic wave apparatus according to the fourth preferred embodiment of the present invention, a particle size of a conductive particle in the conductive film constituting the second electrode layer and the wiring electrode may be smaller than a particle diameter of a conductive particle in one of the electrode for the surface acoustic wave element and the first electrode layer of the electrode pad, which has a smaller film thickness. Consequently, the reliability of the electrical connection of the electrode for surface acoustic wave element and the electrode pad with the wiring electrodes is even more increased.
Regarding the surface acoustic wave apparatuses according to the second to fourth preferred embodiments of the present invention, an electrode for a second surface acoustic wave element that is different from the aforementioned electrode for the surface acoustic wave element may be formed on the piezoelectric substrate, and the electrode for the second surface acoustic wave element may be formed by etching of a metal film. When the electrode for the second surface acoustic wave element is formed by the etching method, it can be simultaneously formed with the first electrode layer of the electrode pad, and the film thickness thereof can be differentiated with ease from that of the electrode for the surface acoustic wave element formed by the lift-off method. Therefore, surface acoustic wave elements having different characteristics can be formed with ease on the piezoelectric substrate.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.